High optical quality (high-Q) whispering gallery mode (WGM) microresonators have been a subject of intense investigation during the last decade because of their strong potential for numerous high-performance photonic devices, including ultra-sensitive molecular detectors and advanced light sources, such as narrow linewidth lasers and comb generators. The unique characteristics of such WGM devices appear to be particularly relevant for mid-infrared (MIR) applications, because of the stronger molecular absorption lines in the MIR, and because of the increasing need for frequency comb sources in this “molecular fingerprint” region. In particular, prior efforts have estimated a sensitivity of a few parts per trillion for the detection of several strongly-absorbing atmospheric and biogenic trace gases in this MIR “molecular fingerprint” region by combining the sensitivity benefits of wavelength modulation spectroscopy with high-Q MIR microsensors in compact “field-usable devices”, facilitating their use for numerous applications in industry, environmental sensing, and agriculture. Such miniature portable “ultra-sensitive” molecular sensors could pave the way for numerous cutting-edge uses in diverse fields and applications such as: (1) breath analysis of patients for healthcare; (2) process control systems (such as ultrasensitive moisture monitoring) in manufacturing; (3) environmental monitoring of industrial pollutants, including hydrogen sulphide and carbon monoxide levels (say on roadsides and in parking structures); (4) the precise monitoring of trace gases that affect the environment, such as CO2 and other greenhouse gases, biogenic emissions from flora and fauna, coastal and oceanic carbon and nitrogen compounds, and geothermal and volcanic emissions; and (5) measurements of emissions from agricultural enterprises including crop growth and livestock farming (e.g., ammonia emissions).
There have also been considerable advances in the development of Whispering-Gallery mode (WGM) microlasers in the past decade as key enablers for numerous high-performance photonic devices, including ultrasensitive molecular detectors and compact narrow-linewidth lasers. Most of the past WGM microlaser developments have focused on visible and near-IR sources.
Whispering-gallery-mode (WGM) microlasers are particularly attractive and popular, largely because of their relative ease of fabrication (as opposed to their electrically pumped semiconductor counterparts, which require much more elaborate design and fabrication processes. Even though optically pumped microlasers have been demonstrated with a broad range of glass and crystal hosts, spherical and toroidal structures, based on the rare-earth-doped silica material system, are the most common, in part, because of the ease of fabrication of such structures via simple melting techniques.
Unfortunately, silica-based microlasers are limited in their operating wavelength ranges to the visible and near-IR because of the strong increase in absorption in silica at wavelengths >2 μm. However, because of the stronger molecular absorption bands in the mid-infrared (MIR), ultra compact, microlaser sources at longer MIR wavelengths are still critically needed for several key applications, notably high-resolution spectroscopy and trace level detection of several important molecular species. Previous mid-IR WGM microlasers have been based on electrically pumped semiconductor sources that are difficult to fabricate and are also limited to operation either at cryogenic temperatures, or in the pulsed mode at relatively short (2.4 μm) MIR wavelengths.